A conventional non-oriented electrical steel sheet generally has a minimum core loss at a grain size of about 150 μm. Thus, a steel sheet of a better grain growth during finish annealing can be desired in view of the product property, simplified manufacturing process, and high productivity.
For example, a preferable grain size may be less than, for example, 40 μm when the steel sheet is subjected to punching for magnetic cores by customers because the accuracy in punching is better when the grain in the sheet is finer. As described above, preferences for core loss and the punching accuracy to the grain size may conflict with one another.
When these conflicting requirements are satisfied, a product sheet can be shipped with a small grain size and may be subjected to stress relief annealing, for example, at 750° C. for about two hours to grow the grains after punching by the user or consumers. Recently, the customers have had strong demands for materials with low core loss. In addition, there may be an increasing need for the product sheets which have a better grain growth during stress relief annealing since a reduction in the stress relief annealing time has been demanded because of improvement in productivity by the consumers.
One of major factors of inhibiting grain growth is the dispersion of fine inclusions in the steel. It is known that the grain growth is more inhibited with larger number and smaller size of the inclusions in the product.
In particular, as presented by Zener, the grain growth deteriorates further with a smaller r/f value that is represented by the equivalent volume radius r and the volume fraction f of inclusions in the steel. Accordingly, it is important not only to decrease the number, but also to increase the size of the inclusions for a good grain growth of the steel.
For preferable ranges of the size and number of inclusions in the non-oriented electrical steel, for example, Japanese Patent Application Laid-open No. 2001-271147, the entire disclosure of which is incorporated herein by reference, describes that inclusions with a size from 0.1 [μm] to 1 [μm] and inclusions with a size of greater than 1 [μm] are contained within a range from 5000 [/mm2] to 105 [/mm2] and a range of 500 [/mm2] or less, respectively, per unit cross-section area.
For example, the number of inclusion per unit cross-section area can be converted to the number per unit volume. The above-indicated ranges can be converted to the range of 5×106 [/mm3] to 1×109 [/mm3] and the range of 5×105 [/mm3] or less, respectively
Inclusions inhibiting grain growth in a non-oriented electrical steel sheet are, e.g., oxides such as silica and alumina, sulfide such as manganese sulfide, and nitrides such as aluminum nitride and titanium nitride.
Highly purified molten steel generally provides a steel sheet free from these inclusions. There are several methods to reduce detrimental effects of the inclusions by adding various elements to the molten steel.
For oxides, a technological progress allows a removal of oxides from molten steel by adding a sufficient amount of Al, a strong deoxidizer, and stirring enough periods to float them up for removal.
For sulfides, in order to remove sulfur from molten steel thoroughly, the methods of adding of some rare earth metals as desulfurizer to fix sulfur in the steel is described in, for example, Japanese Patent Application Laid-open No. Sho 51-62115, Japanese Patent Application Laid-open No. Sho 56-102550, and Japanese Patent Publication No. 3037878, the entire disclosures of which are incorporated herein by reference. Further, for nitrides, the methods of adding boron that lead to the formation of coarse BN inclusion in the steel and the prevention of finer other inclusions are described in Japanese Patent Publication No. 1167896 and Japanese Patent Publication No. 1245901, the entire disclosures of which are incorporated herein by reference.
However, the high purification in the stage of molten steel may not be preferable because of unavoidable increased steelmaking cost. On the other hand, the above-described methods of adding elements are insufficient in improvement of grain growth and core loss in finish annealing or stress relief annealing after punching at lowered temperature and reduced period.
Even when the number density of inclusions is adjusted to fall within the recommended range described in Japanese Patent Application Laid-open No. 2001-271147, the grain growth would likely still be unimproved in some cases where the stress relief anneal is performed at a lower temperature and for a shorter period.
This may be because the size and the number density of inclusions adjusted based on the conventional knowledge are different from the composition, the size, and the number density of inclusions actually inhibiting grain growth as described herein below.